HUD systems typically are used in aircraft to provide pilots with information superimposed onto their forward field of view through the aircraft windshield. The information displayed may be data or symbolic images indicative of flight conditions such as the operating condition of the aircraft, environmental information or guidance information for use in bringing the aircraft to its destination. These images are presented in overlying fashion on the pilot's field of view so as not to interfere with the pilot's view of the background scene.
Although HUD systems may be useful in a variety of applications, known combiners are either undesirably thick and heavy for most applications, or are thin and undesirably highly curved. The thick combiners contain a pair of cooperative lens elements, at least one of which includes an embedded spherical surface coated with a spectrally reflecting thin film. The external surfaces of these thick combiners are flat so as to provide an undistorted view of the background scene. Thin combiners, on the other hand, typically employ a pair of spherical external surfaces, one of which carries the spectrally reflecting thin film. Thin combiners thus typically do not provide the necessary undistorted view of the background scene, especially when the combiner is thick enough to be adequately durable.
What is needed is an optical combiner which is thin and flat, but which contains sufficient optical power to present a collimated image on a relatively undistorted background scene.